A 19-year-old man was arrested Friday at the U.S.-Mexico border in connection with the shooting death of a 17-year-old boy in San Marcos earlier this year, a sheriff’s official said. The alleged shooter was located by U.S. Marshals at the San Ysidro Port of Entry. He was arrested on suspicion of murder in the death of Isaac Shekhayem of San Marcos, who was shot in the head on Los Vallecitos Boulevard just north of state Route 78 on May 15, sheriff’s Lt. Michael Krugh said. Investigators said Shekhayem was in a group of people standing in a parking lot when a vehicle pulled up around 11:30 p.m., Krugh said. A gunman got out and opened fire. Shekhayem was taken to an emergency room by two companions shortly before he died, Krugh said.